Heretofore, for membranes for electrolysis of sodium chloride, for membranes or catalyst layers for polymer electrolyte fuel cells, it has been common to employ a polymer which is obtainable by hydrolyzing a copolymer of a fluoromonomer represented by the formula CF2═CF—(OCF2CFRx)x1—Ox2—(CF2)x3—SO2F (wherein Rx is fluorine atom or a trifluoromethyl group, x1 is an integer of from 0 to 3, x2 is 0 or 1, and x3 is an integer of from 1 to 12, provided that x1+x2>0) with tetrafluoroethylene, or a polymer having sulfonic acid groups obtainable by further converting it to an acid form (hereinafter referred to as a sulfonic acid polymer).
Such a sulfonic acid polymer has a softening temperature in the vicinity of 80° C. Accordingly, the operation temperature of a fuel cell employing such a polymer is usually at most 80° C. However, in a case where hydrogen obtainable by reforming an organic compound such as methanol, natural gas or gasoline, is used as a fuel gas for a fuel cell, if carbon monoxide is contained even in a trace amount, the electrode catalysts will be poisoned, and the output of the fuel cell tends to be low. Accordingly, in order to prevent such a trouble, it is desired to increase the operation temperature. Further, also with a view to downsizing the cooling device for fuel cells, it is desired to increase the operation temperature, and preferably, a membrane for operation at a temperature of at least 120° C. is desired. However, the above-mentioned conventional sulfonic acid polymer has a low softening temperature and can not satisfy such demands. As a polymer having a high softening temperature, a copolymer of a monomer represented by the following formula (y) (hereinafter referred to simply as a monomer (y)) with tetrafluoroethylene, has been proposed (Patent Document 1). In the formula, Qy is a fluorinated bivalent organic group, and each of Ry1 to Ry3 which are independent of one another, is a fluorine atom or a fluorinated monovalent organic group.

Further, Patent Document 2 discloses a monomer represented by the following formula (z) (hereinafter referred to as a monomer (z)). Here, Xz represents a various functional group such as F, Cl, —OC6F5, —CN, —COF, —COORz1 (wherein Rz1 is —CH3, —C2H5 or CH2CF3), —SO2F or —SO2Cl, Rz is a fluorine atom or a perfluoroalkyl group, and Qz is a perfluoroalkylene group which may contain an etheric oxygen atom.

However, with respect to a monomer (z) wherein Xz is —SO2F or SO2Cl, no synthetic example is disclosed, and there is no indication to use a polymer obtained by polymerizing the monomer (z) for fuel cells.
Patent Document 1: WO03/037885
Patent Document 2: U.S. Pat. No. 4,973,714